The invention relates to a leakage compensation arrangement in a control device for a fully hydraulic steering system with a supply connection arrangement having a high-pressure connection and a low-pressure connection, a working connection arrangement having two working connections, a control section between the supply connection arrangement and the working connection arrangement, a control element for activating the control section, a steering member and an auxiliary fluid path with a valve arrangement, through which hydraulic fluid can be supplied or drained off.
A control device of this general kind is known from DE 40 42 151 A1.
In a fully hydraulic steering system, there is no direct active mechanical connection between the control element, for example a steering handwheel, and the steering member, for example the steered wheels or the rudder of a ship. The control of the steering member practically occurs exclusively by means of a hydraulic fluid, which is supplied to a motor via the control section. Thus, the control section firstly controls the direction, in which the steering member is moved. This direction is determined by the movement direction of the control element. Secondly, the control section usually also controls the extension of the movement of the steering member.
Due to unavoidable tolerances in the control device, which are required to ensure that the various parts of the control device can move in relation to each other, inner leakages occur. Because of the inflow or outflow of hydraulic fluid, the allocation of control element and steering member in relation to each other is displaced. This may cause that one position of a steering handwheel, which is at a certain time allocated to straight-ahead driving, will eventually cause a curve driving.
Usually, the person operating the control device will compensate this error. Normally, such an operating person does not steer according to the position of the control element, but according to feeling. Still, however, a certain correlation between the position of the control element and the position of the steering member is desired, particularly when further operating elements are arranged on the control element, for example switches for activating further functions of a vehicle to be controlled by the control device. When these switches move out of the reach of the operating person, this is regarded as disturbing.
In the control device known from DE 40 42 151 A1, valves have therefore been arranged in the auxiliary fluid path, via which hydraulic fluid can be supplied or drained off. For controlling the valves, a control is provided, which is connected with two sensors. One sensor monitors the position of the control element. The other sensor monitors the position of the steering member. When both positions do not correlate with each other, a valve is activated to supply or drain off hydraulic fluid, until the correlation has been achieved again. Even though this procedure has proved its worth, it is relatively expensive.
A similar control device is known from U.S. Pat. No. 5,267,628. Also this document shows an electronic solution, which is expensive to manufacture and difficult to mount.